Food contains carbohydrates which results in minute quantities of acid production. This acid weakens enamel temporarily for about 45 minutes after which enamel reminiralizes itself from saliva. If we brush within 45 mins to 1 hr, we are brushing weakened enamel which will wear off more easily, gradually leading to enamel loss and sensitivity.Hence it is advisable to brush either before meals or after an hour of having meals.